<header>SMTP server response on domain reject</header>

<center><tt>unknown_address_reject_code</tt></center>
<hr>

This parameter specifies the SMTP server response code when a client violates the
<tt>reject_unknown_sender_domain</tt> or
<tt>reject_unknown_recipient_domain</tt> restriction rules that are:<br>
<i>Access denied: Unknown domain for sender or recipient email address (without DNS A or MX record)</i><br>
<br>
Note that the postfix default value used for <tt>unknown_address_reject_code</tt> is
code 450 also used for other errors. This code in 'standard RFC 822' means
'Requested mail action not taken: mailbox busy or access denied' and
ask the sender to try again later (hoping the DNS will answer correctly
later).<br>
<p>
Do not change this unless you have a complete understanding of RFC 822.
</p>

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